I. Field of the Invention
The present invention pertains to the field of satellite communications. More particularly, this invention relates to compensating for the presence of noise in satellite communications.
II. Related Art
In a satellite communication system, one or more satellites receive a signal from a transmitter, such as a wireless user terminal, at one location and forwards the signal to a receiver, such as a ground station or gateway, at some other location. Transferring signals through satellites allows communication systems to provide communication services virtually anywhere. While not inherently inexpensive to construct and launch, satellite systems can also provide relatively cost effective services because very little land-based infrastructure, such as telephone lines, base stations, and cellular towers, is needed to cover very large areas separated out over very long distances. These advantages make satellite systems ideal for a wide variety of communications applications.
In an exemplary satellite system, user terminals communicate with a “gateway” through a satellite. A gateway provides access to other user terminals or to other gateways, and through them, other terminals or services available in a given communication system. Gateways may also be equipped to provide the terminals with access to outside networks or connected communication systems. For instance, the gateway may include an Internet access point so that terminals can access the World Wide Web, for activities such as sending and receiving email, obtaining financial or other information, and so forth, through the satellite and the gateway. Gateways may also connect user terminals to Intranets or other types of networks for business communications and information.
The terminals and gateways transmit and receive radio frequency (RF) signals that can be degraded or disrupted by the presence of noise, potentially preventing or substantially reducing the quality of communications. Signal quality is often measured as a ratio of signal level to noise level, or signal-to-noise ratio (SNR). One source of noise in radio frequency signals is interference among signals, or between signals being transferred by nearby wireless devices. Numerous techniques have been developed to control how and when signals are transmitted so that intra-user or -device interference is reduced or minimized.
One common approach or signal modulation technique used to reduce interference is called time division multiple access (TDMA). In a basic TDMA system, only one terminal is allowed to transmit a signal at any instant in time over a given frequency. Time allocated to signal transfers is divided into frames, and each frame is further divided into time slots. Each terminal that is active during a particular time frame is assigned to one of the time slots. Interference among the terminals in such a system is virtually eliminated or greatly reduced as long as frequency re-use is kept to a minimum or well controlled for adjacent areas.
Another common approach or signal modulation technique used to reduce interference is called frequency division multiple access (FDMA). In a basic FDMA system, each terminal is assigned to a frequency band, or slot. The frequency slots are often separated by guard bands, and terminals use filters so that several terminals can simultaneously transmit in separate bands. Depending on the quality of the filtering provided between frequency slots, interference among the terminals in such a system can be substantially reduced.
Yet another common approach or signal modulation technique used to reduce interference is called code division multiple access (CDMA). In a basic CDMA system, each terminal is assigned a specific code channel or code to be used in spreading or modulating signals, on the forward link, and typically a set of codes for the reverse link modulation scheme. There may be a PN code for spreading and an orthogonal code for channelizing the signals. Many terminals can transmit simultaneously using the same frequency band or overlapping bands. Signal components from a terminal transmission are spread over the frequency band based on the corresponding PN spreading and orthogonal codes. A receiver uses the same codes to reconstruct the transmission.
Depending on the number of codes and the quality of the signal generation and reconstruction, interference among the transmitting terminals in such a system can be substantially reduced.
The three signal modulation or air interface techniques or approaches, TDMA, FDMA, and CDMA, can also be used together in a variety of ways. For instance, groups of terminals can be assigned to time slots, and then within a particular time slot, terminals can be assigned to frequency slots and/or code channels or codes. Using these approaches, large numbers of terminals can be serviced by a single gateway through a satellite.
Another source of noise is called thermal noise. Thermal noise includes various naturally occurring phenomenon, not the least of which is rain. Depending on its intensity, rain can substantially degrade signal quality. One can try to increase the signal gain or power to overcome the degrading condition, but this is not always possible or even desirable. There are limits to the amount of power that can be used because wireless communication systems, especially satellite systems, are power constrained.
You can only adjust power so far before you reach a limit to the either the available power or the handling characteristics or capabilities of the equipment. In this situation, an additional mechanism is needed to overcome the problem. In addition, with power being a finite resource for wireless devices and satellites, applying more power drains that resource, which eventually requires equipment replacement, which is not practical.
Furthermore, high quality high power circuits and controls tend to be more expensive the more power they are required to accommodate, so it is desirable to be able to design systems that can maintain operation under a variety of conditions with lower power requirements.
Therefore, a need exists for noise compensation in a satellite system with little or no increase in interference.